1. Field of the Invention
The present invention relates to a driving circuit and a semiconductor device, and more particularly, to a driving circuit that drives a switching element connected to a main current circuit of high voltage large current. For example, it relates to a driving circuit that drives a high-speed switching element, such as a MOSFET, and particularly, a switching element including a wide bandgap semiconductor. Moreover, it relates to a semiconductor device including the driving circuit.
2. Description of the Background Art
Conventionally, in a case where “a semiconductor switch group represented by a MOSFET” (hereinafter, simply referred to as a “MOSFET”) such as a power MOSFET (a hybrid switch of a MOSFET and a J-FET) is used for power equipment, it is mainly used as discrete components for a switching power supply and the like. To make full use of the MOSFET having high speed performances, a switching speed has been improved.
On the other hand, in a field handling high voltage large current (generally, the rating is 300 V, 100 A, or more), an IGBT has been mainly used and the MOSFET has been rarely used. When the MOSFET is used, its high speed performances are expected. In recent times, upon developing a SiC-MOSFET, since its wide bandgap has suitability for high breakdown voltage, the MOSFET is likely to extend a range of the suitability in a field in which the IGBT has been used. If high breakdown voltage is achieved, an expansion of likelihood of voltage fluctuation simultaneously expands suitability for large current.
An IGBT is a switch that performs a combined operation of a power MOSFET and a bipolar transistor. A turn-off operation of the IGBT has characteristics as described below under general using conditions. During the turn-off operation, bipolar properties become predominant, and negative feedback properties due to a continuation of a collector current and a base width modulation generate strong negative feedback especially on condition that a voltage between a collector and an emitter increases, thereby suppressing the turn-off speed moderately. Without intentionally adjusting in particular, “trade-off between on-voltage and speed” adequate for the conventional main uses of a motor control, an UPS, a CVCF, and the like may be selected to create the state as described above.
On the other hand, the MOSFET target for this present invention does not sufficiently generate the negative feedback due to the properties of the switch itself and almost immediately shows a change in current of the switch corresponding to a control voltage. It also includes the IGBT having properties adjusted closer to a state of the MOSFET in particular and having a turn-off operation similar to that of the MOSFET.
When driving the switch employed for the high breakdown voltage large current, a driving technique different from conventional one is required. In this circumstance, the prior art regarding the MOSFET application of high breakdown voltage large current has been developed (for example, see Japanese Patent Application Laid-Open No. 2004-14547).
In a case of employing the power MOSFET for a large current of 100 A or more, particularly 300 A or more, the current is large for turn-off speed of the MOSFET, so that di/dt as the ratio increases and it is difficult to reduce stray inductance (namely, self-inductance in which an interconnection obtains unintentionally) because the interconnection needs to be increased in a geometrical size to carry the large current. A technique for intentionally reducing the switching speed of the MOSFET is required for reasonable costs.
As a method to reduce the switching speed of the MOSFET, a driving current is basically reduced by increasing a gate resistance (in other words, a charging speed of an input capacitance of the MOSFET is reduced) and a charging voltage increase speed of an input capacitance of the MOSFET is reduced by increasing the input capacitance. However, the application of large current has a constraint on heat dissipation capacity of a heat loss by a switching loss, and in terms of saving energy, it is required to minimize the loss. Thus, such simple method is not sufficient.
In the application of large current, a load for switching is generally inductive, so that in the turn-off switching, a change occurs in current after a change in voltage is almost completed. Since di/dt concerns a current change speed in a latter half of the turn-off switching, the current change speed in the latter half needs to be suppressed without sacrificing a voltage change speed in a first half of the turn-off switching. However, this timing changes under various conditions, and thus it is not easy to predict the timing from indirect parameters.
In this case, the method, which intentionally increases an inductance of an interconnection shared between the main current circuit connected to the source terminal and the driving circuit (hereinafter, referred to as a common inductance) to add the negative feedback, is known. However, to decrease influences of the switching electromagnetic field on the driving circuit in a large current switching, it is preferable that the driving circuit is disposed in a distance from the MOSFET. Under constraints of the present interconnection and heat dissipation technique, an inductance of an interconnection from the driving circuit to the MOSFET (hereinafter, referred to as “a driving inductance”) has a value which cannot be ignored for the inductance value appropriate for the negative feedback in the application of large current, and the driving inductance has a tendency to keep passing the driving current which interferes the negative feedback, thereby resulting in insufficient effects.